It is well known that organic, and in particular cellulosic, materials can be pyrolyzed to produce valuable solid residue products, including charcoal and activated carbon, as well as gaseous and liquid fuels. Exemplary of the prior art that describes such processes is Bowen U.S. Pat. No. 4,145,256; a pyrolysis process specifically adapted for the production of highly activated carbon is disclosed in Bowen and Purdy application for U.S. Pat. Ser. No. 84,294, filed on Oct. 12, 1979, and now issued as U.S. Pat. No. 4,230,602 on Oct. 28, 1980.
In such a process, and depending upon the conditions of operation and the volumes of gases introduced (e.g., air and steam) and generated, there will normally be a significant amount of particulate solids entrained in the offgas stream. Since the desired liquid and gaseous products are obtained from the offgas stream, and since it is necessary that such products be relatively free from such contamination, it is important that these particles be removed. This is, of course, a well-recognized problem, which has in the past been dealt with in a number of ways.
For example, the gas stream may be passed from the reactor into a cyclone separator for the purpose of removinb the solid particles, and then treated in a condenser and demister. However, because pyrolysis offgases contain significant amounts of tars and oils, in addition to the particulate solids, condensation on the walls of any such equipment is considerable, and becomes excessive after relatively short periods of operation. The deposits of condensates and solids must be removed periodically to allow satisfactory operation, necessitating not only significant and frequent periods of downtime for servicing of the system, but also representing an arduous, unpleasant and hazardous task that must be performed manually.
As an alternative to such a "dry" offgas treatment, water scrubbing has also been employed. However, as conventionally practiced that approach also suffers from serious disadvantages, outstanding among which are the need to treat the wastewater to make it environmentally acceptable and to recover from it as much as possible of the valuable organic constituents, such recovery being inhibited to a considerable extent because as much as 60 percent of the condensed organics are in solution. Thus, the separation requires the provision of a wastewater treatment plant, which represents not only a large capital expenditure but also an inconvenient and expensive added operation.
In view of such disadvantages, it has been suggested that the gas stream be cleansed by scrubbing it with the pyrolytic oil. Doing so may obvisously avoid the introduction of, and hence the need to treat, water, and it inherently enables the recovery of at least some of the oil. Furthermore, since the oil itself functions as a natural solvent for condensible fractions of the offgas stream, those fractions of the deposits are readily dissolved, thereby removing them and washing the particles from the interior walls of the system, thus continuously maintaining proper operating conditions without need for regular manual cleaning.
Notwithstanding the advantages of such a procedure, as far as is known there has not heretofore been such an implementation of it as will permit practical operation on a continuous basis. Moreover, it is not believed that there has to date been developed such a system or method, which utilizes pyrolytic oil scrubbing for the production of products of optimal value, and especially enriched gases.
Accordingly, it is a principal object of the present invention to provide a novel, continuous process, and a novel system for carrying out the same, for pyrolyzing a cellulosic material so as to produce pyrolytic oil, a solid residue, and a cleansed, enriched gaseous product.
It is a more specific object of the invention to provide such a process and system in which the pyrolytic oil produced is utilized as the scrubbing medium to remove particulate solids from the gas stream, to thereby avoid problems attendant to the use of dry and conventional water-scrubbing cleansing techniques.
Another object of the invention is to provide a process and a system of the foregoing nature, in which the characteristics of the recycled pyrolytic oil stream may automatically be controlled, to either permit or prevent dehydration of the oil, so as to ensure optimal operation and the attainment of the foregoing objects.
Yet another object is to provide such a novel method and system wherein a portion of the water vapor condensed from the gaseous product may be discharged directly, as a non-polluting wastewater stream.
A further object of the invention is to provide a process and system having such features and advantages, which is also convenient, efficient and relatively simple and inexpensive to carry out and to use.